1. Field of the Invention
The invention relates to a method of manufacturing a semiconductor device comprising a semiconductor element that is situated on an electroconductive plate, wherein a side of the semiconductor element adjoining the electroconductive plate borders on an electroconductive layer containing Au (gold), and the semiconductor element is attached to the electroconductive plate by means of an Ag (silver)-containing organic matrix which is cured by means of a thermal treatment. Such a method is advantageously applied in the semiconductor industry, in particular in the manufacture of discrete semiconductor components. In said application, the conductive plate is in the form of, for example, an assembly of conductors in a so-termed lead frame. After the element has been attached to the plate, one or more electroconductive connections are made between (the upper side of) the semiconductor element and the conductive plate by means of wire bonding.
2. Description of the Related Art
Such a method is known from the abstract in the English language of a Japanese patent specification published under patent number JP A3 41742 on 22-2-1991. In said document, a description is given of the way in which a semiconductor element is provided, on a (lower) side, with an alloyed layer containing Au and Sb, after which it is glued, on this side, to a conductive plate by means of an Ag organic matrix. After curing the silver-containing organic matrix, wire connections are made between the element and the conductive plate. A thermal treatment which is customarily carried out to cure the silver-containing organic matrix normally takes place at a temperature in the range from 150 to 250xc2x0 C. Finally, the device is provided with a synthetic resin envelope.
A drawback of the known method resides in that the adhesion of the semiconductor element to the conductive plate is sometimes found to be insufficient to resist the force(s) exerted on the connection during wire bonding, as a result of which the semiconductor element becomes detached. This applies, in particular, if the semiconductor element is comparatively small, for example if its dimensions are below approximately 400 xcexcm. All this results in rejects, leading to an increase of the cost price. Instead of a conductive glued joint use can be made of a soldered joint. A soldered joint does not have the above-mentioned drawback, but it does have another drawback, namely that, in particular, an inexpensive and hence attractive, conductive plate of copper must first be provided with a silver layer to deal with stresses caused by the substantial difference between the coefficient of expansion of copper and that of the semiconductor element.
Therefore it is an object of the invention to provide a method which does not have the above-mentioned drawbacks, and which is inexpensive, straightforward and results in an excellent adhesion of the semiconductor element on the conductive plate, also if said plate is made of copper, without requiring the application of a silver layer on said conductive plate, and which also results in a very good wire bondability of the semiconductor element after it has been attached to the conductive plate.
To achieve this, in accordance with the invention, a method of the type mentioned in the opening paragraph is characterized in that said thermal treatment takes place at a temperature of at least 350xc2x0 C. Surprisingly it has been found that a gold and (germanium or antimony)-containing layer of a eutectic composition on the semiconductor element results in a particularly strong and properly conducting connection between the semiconductor element and the conductive plate, provided that the silver-containing organic matrix is cured at the surprisingly high temperature of approximately at least 350xc2x0 C. Excellent results have been achieved by subsequently providing the semiconductor element with wire connections to the conductive plate, i.e. the percentage of rejects in this process due to semiconductor elements becoming detached from the conductive plate is very low and much better than the percentage of rejects in the known method. This applies particularly to comparatively small semiconductor elements, such as discrete diodes or transistors, but also to small ICs. Further investigation has revealed that, in this method, the gold and, for example, the germanium that form a layer of melting eutectic penetrate into the silver-containing organic matrix by virtue of, inter alia, the comparatively high temperature treatment. Apparently, this causes the bonding effect of the (cured) silver-containing organic matrix to be improved, while the electric conductivity of the organic matrix remains satisfactory. Very surprisingly it has been found that, in spite of this improved adhesion and high-temperature curing, the organic matrix still remains sufficiently flexible to deal with a difference in thermal expansion between the semiconductor element and the conductive plate, if necessary. The use of eutectic AuSb or AuGe enables the gold and antimony or the gold and the germanium to penetrate to a substantial degree into the silver-containing organic matrix at the temperature used, which ranges, for example, between 350 and 400xc2x0 C.
The best results are obtained if the thermal treatment takes place at a temperature of approximately 400xc2x0 C. This temperature is clearly above the melting point of eutectics of, for example, gold and germanium or gold and antimony, which melt at a temperature of approximately 365xc2x0 C. Connections formed at this temperature are found to be very robust during a subsequent wire bonding process step. The tensile strength at which the connection between the semiconductor element and the conductive plate is interrupted is approximately a factor of 2 higher than that of a connection formed at 350xc2x0 C. At a substantially higher temperature, for example 450xc2x0 C., the properties of the connection deteriorate and the organic components of the silver-containing organic matrix used partly decompose.
Optimum results are achieved if the Ag-containing organic matrix is provided on the semiconductor element via a conductive layer containing eutectic Auxe2x80x94Ge, after which the semiconductor element is pressed down on the electroconductive plate that has been heated to the temperature required for the thermal treatment. By virtue thereof, not only the above-mentioned favorable results are possible, but also the time necessary to form the connection is reduced to a minimum, which is very desirable for industrial-scale applications. The same applies to the use of copper as the material for the electroconductive plate. This material, whose coefficient of expansion differs comparatively substantially from that of the semiconductor element, is more attractive than, for example, an NiFe alloy having a low coefficient of thermal expansion, because said alloy is more expensive than copper and its thermal and electrical conductivity is worse than that of copper. For the silver-containing organic matrix used, use is preferably made of a multi-component mixture on the basis of an epoxy compound. This mixture can be cured in one or more steps. The use of germanium has the additional advantage that this element is less environmentally harmful than, for example, antimony.
Good results are achieved when the curing time is chosen to range between 5 and 50 msec, and preferably the curing time is chosen to be approximately 20 msec. Such a very short curing time enables a large-scale, inexpensive application of a method in accordance with the invention. For the electroconductive plate use is preferably made of a so-termed xe2x80x9clead-framexe2x80x9d, i.e. an assembly of electrical conductors, and the semiconductor element is provided, after it has been attached to the electroconductive plate, with one or more wire connections with one or more conductors that form part of the plate. Subsequently, the semiconductor element, the wire connections and a part of the lead frame are provided with a protective synthetic resin envelope of, for example, an epoxy material.
The invention also comprises a semiconductor device obtained by means of a method in accordance with the invention, which semiconductor device exhibits favorable properties and is economically obtained in large numbers.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.